Divalent cations such as Ca ions have long been known to exert a "stabilizing" effect on the excitable plasma membranes of nerve axons and heart muscle fibers. In terms of the currently accepted paradigm, divalent cations stabilize these membranes (increase the level of stimulus necessary to elicit an action potential) by changing the electrostatic surface potential. They do this by two means. They bind to the membrane and reduce the magnitude of the negative charge and they "screen" these negative charges by forming a diffuse double layer. The biological results are generally interpreted in terms of a combination of the Langmuir adsorption isotherm, the Boltzmann relation and the Grahame equation from the theory of the diffuse double layer. The two critical parameters in these equations are the number of divalent ions bound to the membrane. We propose to test the validity of these equations by measuring both parameters directly. Since Ca ions has no convenient spectroscopic property by which binding may be measured, we will use a paramagnetic divalent cation, Co ions. For our purposes, Co ions is a good analog of Ca ions. Preliminary microelectrophoresis experiments demonstrate that Co ions and Ca ions have precisely the same binding constants to phosphatidyl choline membrane: both ions are divalent and will therefore, exert the same screening effects. We will use phosphorus NMR to measure the binding of Co ions to phospholipids in a bilayer and two different electrochemical techniques to measure the change in electrostatic potential produced by Co ions.